The present invention relates to a shaped body for gas separation having high permeability to gases and excellent performance for the selective separation of gases or, more particularly, to a shaped body for gas separation suitable for obtaining a gaseous mixture highly enriched with respect to the content of oxygen from atmospheric air.
As a trend in recent years, the process of selective gas separation utilizing polymer membranes is highlighted and is replacing the conventional processes for gas separation such as deep-freeze liquefaction and adsorption on an adsorbent more and more from the standpoint of energy saving and prevention of environmental pollution. In particular, a variety of applications are expected for the so-called oxygen-enriching membranes with which a gaseous mixture highly enriched in respect of the oxygen content can be obtained from the atmospheric air including the medical uses and the fields involving combustion of fuels. The development of such a material usable in practical use is eagerly desired.
Oxygen enriching membranes should satisfy several requirements of which the most essential are a high separation factor between oxygen and nitrogen and a high permeability to or velocity of permeation of these gases in order to obtain a high processing capacity. In this regard, although a relatively high separation factor can be obtained with an oxygen enriching membrane by use of certain homogeneous polymeric materials now under development, no practically usable oxygen enriching membrane has yet been developed due to the remarkably low velocity of gas permeation. A relatively high velocity of gas permeation can be obtained by use of a porous shaped body although no sufficiently high separation factor between oxygen and nitrogen can be obtained thereby.
As a means for obtaining a highly efficient oxygen enriching membrane, extensive studies are recently under way of development for the utilization of the technique of plasma polymerization. The principle of this method is that a very thin film having a high separation factor between oxygen and nitrogen is formed by the technique of so-called plasma polymerization on a suitable substrate body having a relatively high permeability to gases which may be a uniform thin membrane substrate or a porous membrane substrate.
The inventors conducted investigations for obtaining a satisfactory substrate body to be provided with a plasma-polymerized film on the surface as an oxygen enriching membrane and have previously proposed an excellent oxygen enriching membrane in both of the gas permeation velocity and the separation factor can be obtained by providing a substrate film or a polymer of a silyl-substituted acetylene compound represented by the general formula R--C.tbd.C--SiR.sub.3, in which R is a monovalent hydrocarbon group, with a polymeric film formed by the low temperature plasma polymerization of a gaseous organosilicon compound such as divinyl tetramethyl disiloxane on the surface thereof. A gas separation membrane of this type, however, has a problem still left unsolved. While it is an important requirement in order to obtain a gas separation membrane of high performance that the plasma-polymerized polymeric film formed on the substrate surface should be uniform and homogeneous as far as possible, namely, such a condition can hardly be satisfied in the plasma polymerization due to the difficulties in controlling the parameters determinant of the conditions of the polymerization. The difficulty in this regard is particularly large when a large surface area of the substrate body should be plasma-treated with sufficient uniformity. For example, the plasma polymerization is greatly influenced by the flow of the monomer gas or the plasma-activated species within the atmosphere of plasma so that the velocity of the polymerization may widely differ from place to place to cause a large variation in the thickness of the plasma-polymerized film. Beside the unevenness in the thickness of the plasma-polymerized film, homogeneity of the film is also a difficult matter to obtain and this problem is particularly serious when the plasma polymerization is continued prolongedly. The reasons therefor presumably include instability of the electric power supply to generate the low temperature plasma, variation in the concentration of the activated species, intermixing of some contaminants with the plasma atmosphere and others. These situations cause difficulties when continuous deposition of the plasma-polymerized film on the substrate surface is desired by continuously passing a continuous-length substrate film of a polymer through an atmosphere of low temperature plasma so that industrial production of gas separation membranes is practically impossible in which reproducibility of the product characteristics is essential along with the productivity.